1. Field of the Invention
The present invention generally relates to a light-shielding blade that is for use in cameras and that is light in weight, high in stiffness, and suitable for high-speed drive.
2. Description of Related Art
A high-speed focal plane shutter is generally composed of a front blade group having four to five light-shielding blades and a rear blade group having four to five light-shielding blades that travel vertically. A light-shielding blade that operates at high speeds must be made of a material light in weight and high in stiffness. Conventionally, an aluminum alloy such as ultra-duralumin, a plastic such as PET film, and a carbon fiber reinforced plastics (CFRP) are used for the light-shielding blade material. Use of light-shielding blades made of such materials realizes a high-speed focal plane shutter of 1/8000 second or 1/12000 second, for example. However, each of these materials has both merits and demerits. Therefore, when assembling a focal plane shutter by combination of different types of blades, the high-speed driving operation is realized by appropriately selecting the material and thickness of each of the different blades. However, the conventionally available materials have reached their limitations because of increasing demands for still higher shutter driving speeds in addition to the requirement for higher qualitative stability and lower cost. For example, the above-mentioned CFRP, which has recently been put into practical use, is light in weight and high in stiffness and therefore is used for some high-speed focal plane shutters as a leading blade material. However, the CFRP has a drawback of making the light-shielding blade thicker than other materials, making it difficult to fabricate blades 20 microns or less in thickness. Further, the CFRP is not satisfactory in planarity, has locally created defects, and is extremely high in cost because of instability in fabrication. In addition, the CFRP is not a material sufficient for providing further higher shutter operating speeds. In addition to the above-mentioned conventional CFRP, the aluminum alloy, and the PET film already in practical use, there have been proposed various materials still under development. For example, Japanese Published Unexamined Patent Application No. Hei 7-333678 discloses a light-shielding blade made of an alloy of Mg--Li. However, this Mg--Li alloy has many problems to be solved such as chemical stability and resistance to corrosion, preventing the Mg--Li alloy from going into practical use. Other light-shielding blade materials that have been receiving attention include beryllium (Be) as disclosed Japanese Published Unexamined Patent Application Nos. Sho 57-141623 and Sho 64-85749.
It is essential for a blade material suitable for high-speed shutter driving operation to be light in weight and high in stiffness. This inevitably points at materials that are as low as possible in specific gravity and high in modulus of elasticity. The weight of a blade is determined by the specific gravity and the thickness of a material used. The stiffness of the blade depends on its thickness. Therefore, the weight of the blade can be reduced by thinning the thickness; however, this leads to reduced stiffness. Generally, a bending moment is used as a scale for representing stiffness. A smaller thickness leads to a lower bending moment. The relationship between the thickness and the bending moment is extremely important for the properties of the light-shielding blade.
Referring to FIG. 7, there is shown a table listing the properties of the blades made of the above-mentioned various materials. In the table, PET represents biaxially oriented polyethylene terephtalate. The aluminum alloy is a kind of duralumin. The CFRP represents a carbon fiber composite material laminated in three layers. The orientation of each layer is at right angles to that of an adjacent layer. The resin pickup relative to the carbon fiber is 40% by weight. The Be represents pure-metal beryllium. For the properties, the table lists the bending moment (g-cm), the specific gravity, and the modulus of elasticity (Kgf-mm.sup.2). Since the light-shielding blade is made of an extremely thin sheet, the bending moment is measured with a Taber's stiffness tester (manufactured by Kumagaya Riki Kogyo Co., Ltd.). To be more specific, the tester measures a load P applied to a piece of sheet 50 mm long L and 38.1 mm wide W to bend the same to an angle of 15 degrees. The value of P.times.L denotes the bending moment. The value of the bending moment is obtained by simulation performed based on actual measurements obtained by varying the blade thickness between 40 and 150 microns. It should be noted that, for CFRP, the maximum values are listed for the bending moment and the modulus of elasticity because CFRP has anisotropy. As listed in the table, PET has the specific gravity of 1.41 and the modulus of elasticity of 540 (Kgf/mm.sup.2). Aluminum alloy has the specific gravity of 2.67 and the modulus of elasticity of 7000 (Kgf/mm.sup.2). CFRP has the specific gravity of 1.50 and the longitudinal modulus of elasticity of 9000 (Kgf/mm.sup.2) and the lateral modulus of elasticity of 700 (Kgf/mm.sup.2). Pure-metal beryllium Be has the specific gravity of 1.84 and the modulus of elasticity of 22000 (Kgf/mm.sup.2). With respect to the bending moment, aluminum alloy and CFRP are 10 times as high as PET, and pure-metal beryllium Be is two times as high as aluminum alloy and CFRP.
As seen from the above-mentioned comparisons, the pure-metal beryllium Be is remarkably prospective as a blade material. However, the Be metals are poor in rolling or calendering process. Therefore, rolling of the Be metals into a thin sheet significantly raises cost, preventing the Be metal sheet from becoming commercially available.